1. Field of the Invention
The present invention relates to selectable oscillation circuits and, more particularly, relates to a selectable oscillation circuit that achieves low power consumption by reducing the amount of current that flows through the inactivated oscillation circuit, when either of two oscillation circuits is inactivated using a selector circuit.
2. Description of the Related Art
Among the mobile communication systems used worldwide, Digital Cellular System ("DCS") is employed in the United Kingdom, Germany, France, and part of Asia, and Global System for Mobile Communications ("GSM"), which was adopted as an European standard method for digital cellular phones in 1982, is employed in Europe, the United States, Africa, and part of Asia.
DCS has 374 working channels and employs Gaussian Minimum Shift Keying ("GMSK") modulation method. In DCS, a frequency band of 1805 MHz to 1880 MHz is assigned to a base station, a frequency band of 1710 MHz to 1785 MHz is assigned to a mobile station, and a 1700 MHz oscillation frequency is assigned to the voltage-controlled oscillator ("VCO") of the cellular phone. GSM has 124 working channels and employs GMSK as a modulation method. In GSM, a frequency band of 925 MHz to 960 MHz is assigned to a base station, a frequency band of 880 MHz to 915 MHz is assigned to a mobile station, and a 900 MHz oscillation frequency is assigned to the VCO of the cellular phone.
Since these two mobile communication systems, DCS and GSM, employ different communication methods, two cellular phones are required in order to subscribe to both DCS and GSM mobile communication systems: a cellular phone adopting the DCS method and a cellular phone adopting the GSM method.
However, since both DCS and GSM methods employ GMSK modulation method, and only their assigned operating frequencies are different, there has been proposed a cellular phone that can be used for both DCS and GSM modes. In this cellular phone, there is provided a selectable oscillation circuit that has a first voltage-controlled oscillation circuit oscillating at 1700 MHz, a second voltage-controlled oscillation circuit oscillating at 900 MHz, and a selector circuit. This cellular phone is controlled so as to select and properly use one of the two voltage-controlled oscillation circuits, in accordance with a control signal from the selector circuit.
In this cellular phone, having both DCS and GSM modes, when mobile communication is performed using DCS method, by causing the selector circuit to activate the first voltage-controlled oscillation circuit and to inactivate the second voltage-controlled oscillation circuit, the oscillation output can be obtained from the first voltage-controlled oscillation circuit. On the other hand, when mobile communication is performed using GSM method, by causing the selector circuit to inactivate the first voltage-controlled oscillation circuit and to activate the second voltage-controlled oscillation circuit, the oscillation output can be obtained from the second voltage-controlled oscillation circuit.
FIG. 2 shows one example of a conventional selectable oscillation circuit in a cellular phone having both DCS and GSM modes. As shown in FIG. 2, the selectable oscillation circuit includes a first voltage-controlled oscillation circuit 21 oscillating at 1700 MHz, a second voltage-controlled oscillation circuit 22 oscillating in the 900 MHz band, a selector circuit 23, a first oscillation-signal output terminal 24, a second oscillation-signal output terminal 25, a selector-signal supply terminal 26, a first selector terminal 27, a second selector terminal 28, a power supply terminal 29, a first frequency-setting voltage supply terminal 30, and a second frequency setting voltage-supply terminal 31.
The first voltage-controlled oscillation circuit 21 includes a first oscillation transistor 211, an emitter load resistor 21.sub.2, a varactor diode 21.sub.3, an inductor 21.sub.4, coupling capacitors 21.sub.5 and 21.sub.6, a direct-current-decoupling capacitor 21.sub.7, a buffer resistor 21.sub.8, base-bias resistors 21.sub.9 and 21.sup.10, and a bypass capacitor 21.sub.11. These circuit elements 21.sub.1 to 21.sub.11 are interconnected as shown in FIG. 2. The varactor diode 21.sub.3 and the inductor 21.sub.4 mainly constitute a first resonator circuit that determines the oscillation frequency of the first voltage-controlled oscillation circuit 21. A predetermined frequency of 1700 MHz, which is obtained by changing a first frequency-setting voltage provided via the first frequency-setting voltage-supply terminal 30 to the varactor diode 21.sub.3, is set as the resonant frequency of the first resonant circuit.
The second voltage-controlled oscillation circuit 22, which has substantially the same construction as the above-described oscillation circuit 21, includes a second oscillation transistor 22.sub.1, an emitter load resistor 22.sub.2, a varactor diode 22.sub.3, an inductor 22.sub.4, coupling capacitors 22.sub.5 and 22.sub.6, a direct-current-decoupling capacitor 22.sub.7, a buffer resistor 22.sub.8, base-bias resistors 22.sub.9 and 22.sub.10, and a bypass capacitor 22.sub.11. These circuit elements 22.sub.1 to 22.sub.11 are interconnected as shown in FIG. 2. The varactor diode 22.sub.3 and the inductor 22.sub.4 mainly constitute a second resonator circuit that determines the oscillation frequency of the second voltage-controlled oscillation circuit 22. A predetermined frequency of 900 MHz, which is obtained by changing a second frequency-setting voltage provided via the second frequency-setting voltage-supply terminal 31 to the varactor diode 22.sub.3, is set as the resonant frequency of the second resonant circuit.
The selector circuit 23 includes a first switching transistor 23.sub.1, a second switching transistor 23.sub.2, and resistors 23.sub.3,23.sub.4, 23.sub.5, 23.sub.6, and 23.sub.7, which are connected as shown in FIG. 2.
The conventional selectable oscillation circuit having the above-described construction operates as follows. In the cellular phone, when mobile communication is performed using the DCS method, a HIGH-level (saturated) selector signal is provided to the selector-signal supply terminal 26 of the selector circuit 23, which switches the first switching transistor 23.sub.1 on and the second switching transistor 23.sub.2 off. At this time, the emitter load resistor 21.sub.2 connected to the first oscillation transistor 21.sub.1 is grounded via the collector-emitter path of the first switching transistor 23.sub.1, which allows an operating current to flow through the collector-emitter path of the first oscillation transistor 21.sub.1. Accordingly, normal oscillation occurs in the first voltage-controlled oscillation circuit 21. The first oscillation signal, whose oscillation frequency is at 1700 MHz, is provided from the emitter of the first oscillation transistor 21.sub.1 via the coupling capacitor 21.sub.6 to the first oscillation-signal output terminal 24. Finally, the first oscillation signal is provided to an application circuit (not shown). At this time, since the second switching transistor 23.sub.2 is switched off, the emitter load resistor 22.sub.2 connected to the second oscillation transistor 22.sub.1 is not grounded, which prevents an operating current from flowing via the collector-emitter path of the second oscillation transistor 22.sub.1. Accordingly, the second voltage-controlled oscillation circuit 22 does not oscillate.
On the other hand, when mobile communication is performed using the GSM method, a LOW-level selector signal (non-conducting) is provided to the selector-signal supply terminal 26 of the selector circuit 23, which switches the first switching transistor 23.sub.1 off and the second switching transistor 23.sub.2 on. At this time, since the second switching transistor 23.sub.2 is switched on, the emitter load resistor 22.sub.2 connected to the second oscillation transistor 22.sub.1 is grounded via the collector-emitter path of the second switching transistor 23.sub.2, which allows the operating current to flow through the collector-emitter path of the second oscillation transistor 22.sub.1. Accordingly, normal oscillation occurs in the second voltage-controlled oscillation circuit 22. The second oscillation signal, whose oscillation frequency is at 900 MHz, is provided from the emitter of the second oscillation transistor 22.sub.1, via the coupling capacitor 22.sub.6 to the second oscillation-signal output terminal 25. Finally, the second oscillation signal is provided from the second oscillation-signal output terminal 25 to the application circuit (not shown). At this time, since the first switching transistor 23.sub.1 is switched off, the emitter load resistor 21.sub.2 connected to the first oscillation transistor 21.sub.1 is not grounded, which prevents the operating current from flowing via the collector-emitter path of the first oscillation transistor 21.sub.1. Therefore, the first voltage-controlled oscillation circuit 21 stops oscillating.
Thus, by providing a HIGH-level or a LOW-level selector signal to the selector-signal supply terminal 26, either the first voltage-controlled oscillation circuit 21 or the second voltage-controlled oscillation circuit 22 is selectively activated. Accordingly, the first oscillation signal or the second oscillation signal can be selectively output.
In accordance with whether the selector signal provided to the selector circuit 23 is HIGH or LOW, one (for example, the first voltage-controlled oscillation circuit 21) of the two voltage-controlled oscillation circuits 21 and 22 is activated and the other (for example, the second voltage-controlled oscillation circuit 22) is inactivated. In this case, when the second switching transistor 23.sub.2 is switched off, since no operating current flows via the collector-emitter path of the second oscillation transistor 22.sub.1, there is no power consumption due to the operating current flowing via the collector-emitter path. However, regardless of whether or not the second oscillation transistor 22.sub.1 is active, some bias-setting currents continue to flow via the base-bias resistors 22.sub.9 and 22.sub.10, and the internal battery of the cellular phone is drained quickly.